This invention relates to apparatus for converging three in-line beams of a cathode ray tube.
Color display systems such as utilized in color television receivers include a cathode ray tube in which three electron beams are modulated by color-representative video signals. The beams impinge on respective color phosphor areas on the inside of the tube viewing screen to reproduce a color scene as the beams are deflected to scan a raster. To faithfully reproduce a color scene the three beams must be substantially converged at the screen at all points on the raster. The beams may be converged at points away from the center of the raster by utilizing dynamic convergence apparatus or self-converging techniques, or a combination of both. No matter which arrangement is utilized to achieve convergence as the beams are deflected, some provision must be made to statically converge the undeflected beams at the center of the screen. Static convergence devices are necessary because tolerances in the manufacture of electron beam gun assemblies and the assembly of the electron gun into the picture tube neck frequently results in a static misconvergence condition.
U.S. Pat. Nos. 3,725,831 and 3,808,570 disclose static convergence assemblies for use with a cathode ray tube which produces three in-line beams. The assembly provides four-pole and six-pole magnetic fields for moving the outside two beams in opposite and in the same directions, respectively, with substantially no effect on the center beam. The strength of the fields produced and the direction of movement of the beams is controlled by rotating about the neck portion of the cathode ray tube a pair of four-pole members with respect to each other and in the same direction, respectively, with similar movement of a pair of six-pole members, which members have equidistantly spaced poles.
The above arrangements can satisfactorily statically converge three in-line beams of a color television picture tube. However, even with the poles of the various magnetic members oriented to produce predetermined field strengths and directions when initially mounted on the tube neck, variations in the initial observed beam landing pattern from one tube to another as described above prevents the operator from following an efficient procedure to statically converge the beams. This set-up problem occurs whether the magnetic members are postiioned by hand or are controlled mechanically by motor driven gears which engage gear teeth on the members, the motor being controlled by suitable switches accessible to the operator. The problem is accentuated when the beams are only slightly misconverged because only a little movement of the members in the wrong direction can produce an even greater misconvergence. The end result can be added cost of manufacture due to a relatively long set-up time or a less than optimum convergence condition, or both.